The invention relates to the field of radio phone systems. A subscriber in a radio phone system, i.e., a radio unit or a subscriber station, for instance a radio phone or some other means of communication, may be registered in a radio network or system, whereby it is registered in the radio network via system and traffic channels maintained by the base stations of the radio network.
In addition to the system channels in radio phone systems, so-called direct mode channels can also be used in connection with a radio system, i.e., direct mode operation is applied. Radio units or subscriber stations using direct mode operation do not communicate directly with the radio network or its base stations. Direct mode channels are frequencies at which radio phones or other means of communication are able to communicate directly with each other without the system.
Direct mode channels are typically used in situations where, for instance a group of portable radio phones communicate with each other at such a distance from the base station that system channels cannot be used.
Another important way of using direct mode channels is to increase the capacity when the traffic in the system increases fast in some part of the service area of the system, for instance, in some point-like part of the radio network.
A direct mode channel is referred to as a direct or simplex channel, or a simplex connection. A direct mode channel is a channel which is not typically used by the system. It may be, for instance, a channel of the breadth of the channels of the system, for instance 12.5 kHz or 25 kHz. Among the radio phones operating on a direct mode channel, the transmitting station has tuned its transmitter to the channel and transmits speech or data information. The other radio phones set to direct mode operation have tuned their receivers to the same channel, whereby they are able to directly hear the transmission.
Operation on a direct mode channel may take place on the analog or digital modulation principle. A radio phone transmitting on the channel may also transmit signalling information, such as information on rights of use and priorities or on the group operating on the channel. On the direct mode channel, an encryption may be carried out or plain speech can be transmitted.
Radio units or subscriber stations using direct mode operation communicate with other radio units on a direct mode channel without necessarily being in direct contact with the base stations of the radio network.
One form of a direct mode channel is a direct mode channel equipped with a repeater. Such a direct mode channel includes a separate repeater station in addition to the subscriber stations which forwards traffic between the subscriber stations using the direct mode channel. A repeater station is one type of radio unit. The traffic on the direct mode channel, thus, takes place on the semiduplex principle. The subscriber stations using direct mode operation can also communicate with the radio network via repeater stations.
The random access method used in prior art radio phones capable of using direct mode operation is very simple. It is entirely based on the pushes of a tangent performed by users. When the user of a radio unit depresses his tangent, i.e., the push-to-talk button (PTT), the radio unit transmits its signal to the direct mode channel, i.e., the radio path. It is obvious that a signal given by a speech detector can correspond to a push of a tangent.
The problem with the solution according to prior art is that if one radio unit sends out a transmission simultaneously with another radio unit, the signals of both radio units are usually destroyed or corrupted, and the transmission of neither radio unit reaches its destination.
Naturally, due to the above-mentioned reason, the efficiency of channel use is not high in prior art solutions because the systems waste radio path capacity.